Synthesis gas has various impurities such as tars, H2S, NH3 and particulates. Tars, commonly defined as polynuclear aromatic compounds formed in the pyrolysis of carbonaceous materials such as wood, coal, or peat, are responsible for operational problems such as plugging of process lines and fouling of heat exchange surfaces which results in reduced process efficiency and plant shutdowns. Tars have the propensity to act as coke precursors resulting in catalyst deactivation downstream of the gasifier. Additionally some components of tars are known carcinogens. Hence, it is important to remove tars from synthesis gas streams for the economical conversion of synthesis gas to value added products.
The concentrations of tars can vary depending upon feedstocks, gasifier type and operating conditions. Most downstream conversion processes and equipments have zero or very low (in ppb range) tolerance for tars. Although catalytic removal of tars is the simplest and the most economical method, there are no commercialized low temperature (<500° C.) tar removal catalysts even after continued 25 years of research and development efforts. Catalysts currently used in the art require temperatures of at least 600° C., preferably 800° C. which requires heating and expensive equipment. By taking the synthesis gas straight out of the generator absent any additional heating additional costs and machinery are not required.
There exists a need to find a simultaneous tar and sulfur removal catalyst that exhibits: 1) sulfur tolerance for tar removal; 2) resistance to coking; 3) ability to withstand high temperatures and reducing environment; 4) ability to work in the presence of NH3, HCl and some heavy metals; and 5) attrition resistance.